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When rain falls in natural, undeveloped areas, storm water is able to permeate the soil and return to the water table below the surface. Developed areas have a greater concentration of impervious surfaces such as roads, sidewalks, buildings, and parking lots, which slow or prevent water from permeating the soil. This runoff water must be collected and redistributed to ensure safety, access and to reduce the environmental impacts associated with urban development.

Natural conditions limit runoff
Natural conditions limit runoff
Development increases runoff
Development increases runoff

In developed urban and suburban areas, enclosed drainage systems are often used to collect and transport storm water during and after rain events. When rain falls onto an impervious surface such as a paved road, sloped pavement directs the water to a gutter. The gutter directs the water along the curb line to a series of intermittent inlets where it can enter the collection system below the pavement. A system of pipes below the pavement moves the collected stormwater offsite to creeks, streams, or other bodies of water in the surrounding hydrologic environment. Enclosed drainage systems can be effective and exist in virtually every urban area but they are expensive to build, rebuild and maintain and can cause flash flooding and erosion where the fast-moving stormwater exits the system.

Developed area
How can we make this...
Natural area
function more like this?

In a natural system, rain water percolates into the soil and moves slowly to the nearby stream. In an area that is paved and piped, water moves much more rapidly to the nearby streams, thus causing a rapid rise in water level. Examples of damage to stream banks of Fishpot Creek in St. Louis County can be seen at the Soil and Water Conservation District website.

Studies conducted by the Center for Watershed Protection have determined that when impervious surfaces in a watershed exceed 10 percent of the land, water quality in streams begins to decline. At 20 percent impervious surfaces, stream habitat is typically highly degraded, and fish populations are in decline. Stream water quality may not be suitable wading or swimming.

Impervious surfaces from parking lots, roof tops, roads, and sidewalks usually exceed 20 percent of the urban landscape. This factor alone accounts for the poor quality of streams in most urban communities. Pollutants, including crank case oil, pet waste, trash, and other debris are picked up by water flowing off imperious surfaces and carried through the gutters and outlets directly to local streams. Under US EPA Water Quality Phase II Stormwater requirements, communities are implementing new plans to reduce pollutants from non-point sources.

Low Impact Development (LID) is an increasingly popular method of stormwater management that decreases runoff in developed areas by increasing surface permeability and seeks opportunities to store or utilize stormwater on site, as shown in the images below. Some approaches to low impact development include underground storage of stormwater, permeable pavement, rain gardens, and downspouts that empty into landscaping rather than the street.

LID technique
Credit: CH2M HILL
LID technique
Credit: CH2M HILL

The biggest challenge facing the implementation of LID techniques is the lack of design standards and regulation.

Downspout into landscaping
Credit: CH2M HILL

Decision-makers may be reluctant to employ techniques that are not yet considered common. The economic and environmental benefits associated with LID techniques, however, make this a challenge worth overcoming.

The images at right show a landscaped island that provides both a "do" and "don't" in terms of LID strategies. A downspout from the roof above is channeling rainwater directly into the landscaping instead of directing it through an enclosed stormwater management system. This strategy reduces stormwater capacity requirements while reducing the maintenance associated with landscaping. Below, the curb and gutter system used for the landscaping island however, prohibits stormwater from being absorbed by the landscaping, which is a missed opportunity. Placing intermittent curb cuts around the island would allow some rainwater to enter the landscaped island.

Landscaped area to increase permeation
Credit: CH2M HILL

Eliminating the curb and recessing the landscaped area would allow even more stormwater to permeate the natural surface, further minimizing stormwater capacity requirements. These simple, effective LID strategies can have significant environmental and economic benefits.

Drainage for Downtown Main Streets:

Effective stormwater management is imperative. Flooding of thoroughfares, bike paths, sidewalks, and adjacent properties can create safety and access issues as well as significant environmental and property damage.

Consider a combination of conventional and LID stormwater management. Combining these methods can be especially effective in retrofit situations where existing enclosed stormsewer systems are unable to handle the additional runoff created by new development. In this situation, the LID method can be much more cost-effective than rebuilding the storm sewer system.

Design for bicycles. Because bicycles are often expected to use the rightmost edge of the travel lane (a bike lane, paved shoulder, or wide outside lane), they may encounter various obstacles, such as manholes and grates, which are commonly used along the curb as part of enclosed drainage systems. It is important to consider the needs and safety of bicyclists when planning the location and design of these items. Bicycle-friendly grates and manhole covers must be used along designated bicycle lanes and routes, and are strongly encouraged along any corridor where bicycle traffic exists.

Recommended LID techniques include:

  • Pavers
    Credit: East-West Gateway
    Reduce the amount of impervious surface area by encouraging shared parking and using pervious pavement where appropriate. Shared use of parking (e.g. daytime use for offices, nighttime use for a movie theater or restaurants) is a strategy which can be used to reduce the amount of space needed for parking, decrease impervious surface area, and reduces stormwater management requirements. Pervious pavement can also be used in parking lots to further reduce the impervious surface area. Pervious pavement is more permeable than regular pavement and allows stormwater to more easily drain through to the soil. See the EPA Porous Pavement Fact Sheet for a detailed breakdown of the pros and cons of pervious pavement, or visit perviouspavement.com for more information.
  • Use LID techniques to add to the aesthetic environment. Landscaping, green roofs, and rain gardens can help to beautify downtown areas while providing valuable stormwater advantages. Detention ponds are often used to manage stormwater in residential areas and employment districts. They can enhance many types of places such as pocket parks, trails, and greenways.
  • Parking Lot Island
    Credit: East-West Gateway
    Use collected stormwater to maintain landscaping. Opportunities to use LID strategies to supplement enclosed drainage systems are often overlooked or underutilized. For example, place is often placed higher than the surrounding pavement, thus limiting its ability to absorb runoff, as in the image at right.
  • Set the precedent for LID stormwater management. Consider showcasing the approach to stormwater management as a feature of the project. Educating the public and becoming an example for future projects is one way to further establish LID techniques.

LID State of the Practice Example: 12th Avenue Green Street Project, Portland

Portland 12th Avenue Project
Credit: City of Portland/ASLA

The 12th Avenue Green Street Project in Portland, Oregon is a fantastic example of the successful use of LID stormwater management techniques. The American Society of Landscape Architects recently awarded the project a General Design Award of Honor (2006), and stated that the project was “…the best example of this type of work we’ve ever seen.”

The initiative was part of a thoroughfare retrofit project started in 2005. The project uses stormwater planters to collect, treat, and distribute stormwater from the street, bypassing the existing enclosed drainage system.

The planters aesthetically enhance the streetscape, while providing excellent stormwater management, as shown at right. For more information about the project, visit the 2006 ASLA Professional Awards Page.